Typically, test systems that use switches such as electromechanical relays to connect test instruments to DUTs have many more device terminals than instruments, and a given DUT only has a few terminals. Therefore, any given test typically has many unused pins. These unused pins are still connected to DUT terminals, but the pins are not driven by instruments during the test. Each unused pin has capacitance due to cables and DUT terminals. For a variety of reasons relating to DUT arrangement, there may be undesired parasitic connections between the test pins and unused pins, which can cause these unused pin capacitances to be charged up to high test voltages.
If the voltage on an unused pin is not discharged before initiating the next test, and the pin is used in the next test in a test sequence, as the relay opens or closes, the energy stored in this capacitance will suddenly discharge through the relay contact. This is called “hot-switching.” Hot-switching can deteriorate the relay contact, leading to early relay failure.
Ordinarily, an instrument could discharge this capacitance before running a test. However, in such a switch system there is no way of connecting an instrument to a charged pin without causing the relay to hot switch. Also, there is no easy way to know that the other side of the relay is at a voltage sufficient to induce hot switching.
Since the energy stored in an unused test system cable or DUT capacitance increases in proportion to the square of the voltage, and this energy is directly responsible for damaging the relay contacts, this is an important problem in higher voltage test systems, which are becoming more widely used to test high voltage devices.
Embodiments of the invention prevent hot-switching without degrading low current performance, and address other limitations of the prior art.